1. Field of the Invention
This invention relates to an electromagnetic forming method by use of a driver. This forming embraces a geometrical correction which entails substantially no dimensional change. The term "driver" as used regarding this invention means a highly electroconductive material which is formed in conjunction with a workpiece.
2. Description of the Prior Art
When the electric current flowing through a conductor is varied, the intensity of the magnetic field generated around the conductor is also varied. When another conductor exists in the magnetic field so varied, an induced current flows through that conductor in the direction impeding the variation of the magnetic field. Consequently, the electric current is affected by the force (Lorentz force) according with Flemming's left-hand rule. The method of forming an object by the use of this force is called electromagnetic forming. To be specific, when electric energy is supplied to a primary coil, an induced current is produced in a secondary coil (workpiece) and the repulsive force consequently generated between the primary coil and the workpiece effects desired forming of the workpiece.
Electromagnetic forming is characterized in that the forming is effected at an extremely high speed of within 1 ms, the forming force is allowed to act on the workpiece without contact, the forming can easily be controlled and automated as compared with any other high-energy rate forming method, and the forming can be carried out with a simple metal die without inevitably requiring a male die. Electromagnetic forming is capable of bulging or swaging a tubular material, fastening a flange around a tubular material, boring holes in a tubular material, forming a plate-like material. (High-Velocity Forming of Metals; Revised Edition, Manufacturing Data Series, The American Society of Tool and Manufacturing Engineers, 1968.)
Heretofore, in electromagnetic forming of a workpiece of such material as ferrous metal (stainless steel or carbon steel), titanium alloy, or magnesium alloy which has high electric resistance and/or transforming resistance and, therefore, is difficult to work or of such material which has an extremely small thickness and, therefore, is not amply affected by electromagnetic force, it has been customary to oppose a highly electroconductive driver of aluminum or copper as a secondary coil to a primary coil used for the forming. When a tubular workpiece is to be bulged, for example, a tubular driver smaller in diameter than the workpiece is inserted in the central hole of the workpiece with no gap therein. When a tubular workpiece is to be swaged, a tubular driver larger in diameter than the workpiece is wrapped around the workpiece. When a plate-like workpiece is to be formed, a plate-like driver is held fast against the workpiece. Then, the primary coil is opposed to each of the drivers mentioned above. (Electromagnetic Forming (EMF), Metals Handbook, Vol. 4 FORMING, 8th EDITION (1969), American Society for Metals.)
The wall thickness of the driver cannot be reduced below a certain level. The driver is constructed in a thickness of about 0.5 to 0.8 mm, for example, so as not to be deformed easily. Owing to its construction, therefore, the driver has given rise to various problems as described below. Since the driver itself is relatively thick and highly rigid, the electromagnetic forming requires a large amount of energy not only for the formation of the workpiece but also for the deformation of the driver itself. Thus, electromagnetic forming has entailed low energy utilization efficiency. To preclude occurrence of electric discharge between the workpiece and the driver during forming, the workpiece and the driver are required to be held in tight enough contact to preclude occurrence of a gap therebetween. Since dimensions of the driver cannot easily be changed, the driver lacks the flexibility required for meeting possible dimensional variation among the workpieces. Whenever a given workpiece has a different size or shape, it becomes necessary to prepare a new driver which conforms to the size or shape of the workpiece. Since electromagnetic forming is a method suitable for small-lot production of a wide variety of articles, the drivers used are required to be produced in small lots, which leads to an increase in the cost of production. There are times when workpieces make it virtually impossible to attach such drivers thereto because of their size, shape, etc. After completion of the forming, the drivers must be removed from the formed workpieces. Depending on the shape, for example, of the finished workpiece, the removal of the drivers not infrequently proves extremely difficult because all the drivers have thick walls and relatively high strength. Moreover, the drivers are destined to be discarded after use instead of being put to re-use. Since they are produced with thick walls, the cost of their material is so high as to increase the cost of production.
The present invention has issued from efforts directed to eliminating the drawbacks of the prior art described above. An object of the invention is to provide a method of enabling a workpiece of high electric resistance and/or high deforming resistance or workpieces which are difficult to electromagnetically form to be easily, efficiently, and economically formed or corrected.